The human body is tuned to the rotation of planet Earth, our cells responding to the passage from darkness to sunlight like the petals of a flower unfolding to an inviting sky.

Our circadian rhythm prepares us for the day by adjusting hormone levels and blood pressure, among other changes, as we sweep our feet out of bed. Too much artificial light as we sleep, however, might throw that rhythm out of kilter.

A study out of Northwestern University found that sleeping in a room with just a dim light, like a glowing TV or streetlights shining through blinds, might hurt our cardiovascular health. Its authors examined how the bodies of a group of volunteers reacted to sleeping in both dark and dimly illuminated rooms.

You might be asleep as that TV flickers. But your brain is being fooled by what is, in essence, an artificial sunrise.

Our heart rate increases as the autonomic nervous system is activated. And researchers found that our insulin resistance increases as well, disrupting glucose metabolism and increasing blood sugar levels.

All of this, of course, is bad news for the heart. And it adds to previous research showing that sleeping in moderate light is associated with being overweight and obese.

Scientists estimate 40% of us sleep with either the TV on or while light shines from other sources. The problem is particularly acute in urban areas.

Experts suggest we wear eye masks to bed. Keep light sources near the floor. Amber or red/orange light is less stimulating to the brain than white or blue light.

And switch off the TV when you start to get sleepy. It might improve your health — and your electric bill, too.